I was going to reply with something along the lines of "modern filesystems should already have meta-data indexes".... because I thought I'd read about that in connection to ext4, btrfs and/or zfs. Now I'm no longer certain ... all I came up with was the following, which might be of interest:
BabuDB (related to extreemfs.org): http://dl.acm.org/citation.cfm?id=1849822
Spyglass: http://www.ssrc.ucsc.edu/pub/leung09-fast.html
And more from the same researchers: "Scalable File System Indexing": http://www.ssrc.ucsc.edu/proj/fsindexing.html
Perhaps I'm thinking of reiser4?
See also:
TokuFS: https://www.usenix.org/conference/hotstorage12/workshop-prog... https://github.com/esmet/tokufs
I'm still certain I've seen talk of indexing metadata, and I thought it was in an actual, open, system...